Devices are known that are equipped with pacing and sensing circuits associated with the atrium and the ventricle, that can operate according to two known operating modes, DDD or AAI (the AAI mode being a DDD mode having a lengthened atrio-ventricular delay). These devices may be equipped with a mode called “DDD-AMC” or “AAISafeR”, ensuring an automatic mode commutation (switching) from DDD to AAI and conversely. The basic operating mode of a DDD/AAI pacemaker is an AAI mode—or more precisely a “pseudo-AAI” mode—with a single chamber atrial pacing (AAI mode stricto sensu), and a monitoring of ventricular activity. This operating mode is maintained as long as atrio-ventricular conduction is normal, that is, as long as each atrial event (either an atrial detection, corresponding to a spontaneous activity, or an atrial stimulation, corresponding to a paced event) is followed by an associated ventricular detection.
In certain circumstances, however, atrio-ventricular blocks (“AV blocks” or “AVB”) may appear, leading to a temporary disorder of depolarization of the ventricle. In this case, as long as several conditions are met, the pacemaker automatically commutes to an automatic DDD mode, with parameters that are optimized for this situation of a temporary AV block. After the disappearance of the AVB, there is a re-establishment of atrio-ventricular conduction, and the pacemaker automatically commutes back to AAI mode, as long as several other conditions are met. Such a commutation between DDD and AAI operating modes, is notably described in EP-A-0488904 and its counterpart U.S. Pat. No. 5,318,594 (commonly assigned herewith to ELA Medical), and EP-A-1346750 and its counterpart U.S. published application 2004/010292 (commonly assigned herewith to ELA Medical).
The present invention is based upon some observations that have been made while actually following-up patients implanted with DDD/AAI pacemakers with the aforementioned known automatic mode commutation features. Indeed, it has been observed that such devices provide an insufficient specificity for sensing ventricular disorders, such that, in some cases, there are inappropriate mode commutations towards the DDD mode. Indeed, the basic principle for such a device is that when it is operating in AAI mode, any ventricular detection maintains the device in AAI mode (i.e., inhibits commutation to DDD mode), except when such conditions permit a suspicion of an AVB appearance.
The device considers there is an AVB when several criteria are met, revealing a conduction that is actual, but delayed (first degree AVB), or some P-waves that are no longer conducted (second degree AVB), or that are totally blocked (complete, or third degree AVB).
Commuting to DDD mode also can be triggered by the diagnosis of a ventricular pause, i.e., when the interval between two consecutive ventricular events is longer than a specified delay. Detection of a spontaneous ventricular depolarization, and the calculation of the time interval since the previous atrial event, are therefore essential in the diagnosis of AVB, and thus for determining potential commuting to DDD mode.
Notably, after delivering an atrial pacing pulse, the device applies to the sensing circuit, a period called “safety window”, typically 100 ms after an atrial pacing pulse such that if a depolarization is detected before the end of the safety window, it is not taken into account by the device; indeed, due to the very short delay separating this sensing from the previous atrial pacing, it could likely be confused with the sensing of an electric artifact, associated with the recovery time of sensing amplifier for instance. For this reason, in the devices well known in the art, such a depolarization is systematically ignored and is not considered as the end (or beginning) of a ventricular cycle, notably for the management of various intervals (escape interval, atrio-ventricular delay) and management of potential commutation from AAI mode to DDD mode.
But, in some cases, such sensing can be related to an actual ventricular activity, the short AV delay being possibly explained by:                Either a systemic dysfunction, notably consecutive to a defect in sensing a spontaneous atrial event, or        a true atrio-ventricular asynchrony: pathologic lengthening of AV delay, junctional rhythm, chronotropic incompetence with acceleration of ventricular rhythm, etc.        
In the absence of any verification of the true nature of the sensed event within the safety window, the device considers, as a safety measure, that there is an atrio-ventricular defect, in such a way that occurrence of several events of this type may lead to commuting to DDD mode.
Hence, other than the non-detection of a potential ventricular disorder that would be responsible for this situation, the device operation is modified by a false diagnosis leading to an inappropriate commutation to DDD mode. Though DDD mode operation commonly has no deleterious effect to the patient, such a commutation is useless and prevents spontaneous atrio-ventricular conduction, and is therefore generally less desirable.